The present invention relates to methods which quantify the complexities of underground reservoirs and evaluate the reliability of forecasts of oil and/or gas production from such underground reservoirs.
Subsurface evaluations of oil and gas reservoirs are always subject to limits imposed by data quality and availability, resources, timing and costs. As a consequence, decision making should include consideration of the risks and uncertainties associated with an evaluation. Decision Analysis (DA) methods for evaluating and communicating the risks in terms of outcomes, given some defined probability distributions, are intended to be unbiased and accurate. However, comparisons of forecast and actual outcomes for projects typically show that more projects outcomes are below the median forecast (P50) values than above, indicating that in general the input probability distributions used are inaccurate and biased.
Evaluating the potential production of hydrocarbons from an underground reservoir is important in determining the economic viability of an oil- or gas-field development scheme. Several tools exist which assist in making such evaluations and predictions of oil or gas-field performance. These tools also help in deciding how best to develop a field and how to plan for contingencies related to the uncertainties in the knowledge of the subsurface characteristics of the field.
One such tool for assessing the difference between actual and predicted outcomes for a field development is a software application known as GEOSCORE(trademark). GEOSCORE utilizes a method for quantifying anticipated changes in field reserve estimates based on an assessment of subsurface complexity. GEOSCORE assesses scores, on a 1-5 scale, from nine descriptive categories to determine an overall GEOSCORE complexity value. For a particular descriptive component, a score of 1 is associated with reservoirs which have a minimal degree of complexity while a score of 5 implies that the reservoir has a very high degree of complexity for that component. A summation of the nine scores provides the GEOSCORE complexity value. Analysis of GEOSCORE results has shown that fields with relatively low GEOSCORE complexity values are often underestimated in terms of actual production capability. Conversely, the production capability of fields which have very high complexity values often are overestimated due to the adverse effects that complexity can have on the actual producibility of the field. As published, the GEOSCORE estimates are based only on subsurface characteristic components and do not take into account effects on producibility relating to dynamic (reservoir engineering) components which depend on the development scheme selected for a field. Furthermore, no consideration is given to the certainty in the estimates of subsurface characteristic components.
Another tool which can be used to estimate the subsurface complexity of an oil- or gas-field is a software package referred to as GEOFACTOR which was developed by RDS (United Kingdom) and published as a tool via their web-site. This program also suffers from the same shortcomings as GEOSCORE, i.e., no accommodations are made for the chosen developmental schemes or the uncertainty in estimates made for the subsurface characteristics.
Continued widespread and variable inconsistencies between predicted field production and reserves and actual outcomes show there is a need for methods, software tools and analyses which can assess the confidence which should be placed in probabilistic reservoir performance predictions. The present invention is intended to address this need.
The present invention includes an evaluation tool which is intended to help improve the assessment of the reliability of probabilistic reservoir production forecasts. The tool provides an indicator of the reliability of the forecast production profiles and reserves estimations. Moreover, a list of the uncertainties in the subsurface evaluation is ranked by likely impact on the forecasts. The present invention uses simple interactive tools to capture and communicate a broad xe2x80x98snapshotxe2x80x99 of a project""s subsurface complexity and the technical maturity of the evaluation, and to highlight the potential impacts that imperfect knowledge of the subsurface may have on the field development outcomes.
In the present implementation, 89 questions have been defined to capture the key subsurface characteristics and development scheme components of a particular reservoir and development project. A key feature of the analysis is the incorporation of uncertainty into the estimates of the reservoir components used to characterize the reservoir and production development scheme(s). A user""s assessments of the input values for the key components and the uncertainty or confidence of these assessments are captured during the analysis via novel xe2x80x9cGraphic Feedbackxe2x80x9d displays of implied probability distributions.
The present invention provides a method and a computer-readable medium containing executable code for computing an evaluation index for an evaluation of a reservoir. The method comprises the steps of:
(a) making estimates of reservoir components which are associated with a reservoir evaluation;
(b) making estimates of confidence values for the estimates of the reservoir components; and
(c) computing an evaluation index for the reservoir evaluation based upon at least one of the estimates of the reservoir components and the estimates of the confidence values.
The evaluation index may be a complexity index, a confidence index or a technical maturity index. Ideally, the method further includes determining complexity scores associated with the estimate of the reservoir components and also determining confidence scores associated with the estimate of the confidence values. An overall complexity index can be computed from the complexity scores and an overall confidence index can be calculated utilizing the confidence scores.
The estimates of the reservoir components and the confidence values may be made in response to literal and numerical descriptions. Further, estimates of the confidence values may be made in cooperation with a graphic feedback display which depicts at least one probability curve dependent upon the relative level of an estimated confidence value.
Complexity look-up tables may be used to establish the relationship between the estimates of reservoir component complexities and their associated complexity scores. These relationships in the look-up tables are preferably established by a team of experts. Further, ideally these lookup tables can be updated using neural networks or other experience-based knowledge updating schemes. The technical maturity index (TMI) for the reservoir evaluation may be calculated which is a function of the confidence values for the reservoir components.
The TMI is established by calculating a sum of confidence measures weighted by the TMI weights. The TMI weights relate the significance of the confidence of knowing the value for a reservoir component to the predicted outcome of a reservoir evaluation, such as a production profile. These TMI weights are preferably established through the use of expert-determined relationships. In some cases, the weight is set to zero when there is believed to be virtually no impact of uncertainty in an estimate of a reservoir component on a predicted outcome. In other cases, there may be significant dependency between the certainty of the estimate of the reservoir component and a predicted outcome, which is reflected by the TMI weight.
Technical maturity, complexity and confidence indices can be established for a field containing several reservoirs. The TMI can be useful in determining the probability that a field will produce in accordance with predictions. Comparisons between indices in different fields can be useful when ranking a project and as an input to decision-making.
It is an object of this invention to provide both a method and computer readable media containing executable code for carrying out the method which calculates both static (subsurface characteristics) and dynamic (petroleum engineering) complexity and confidence indices, and furthermore, an overall Technical Maturity Index for a subsurface reservoir evaluation and an associated development scheme.
Another object is to highlight the key factors (xe2x80x9creservoir componentsxe2x80x9d) impacting the assessment of the Technical Maturity of a reservoir or field evaluation.
Yet another object is to provide an auditable trail of how a field""s technical evaluation progresses over time, and which uncertainties are identified as crucial at the time of the evaluation.